Passive optical network (PON) is a one-point to multi-point optical fiber access network, which includes optical line terminals (OLT) at central office end, optical network units (ONU) or optical network terminals (ONT) at user end and an optical distribution network (ODN). With the development of PON technologies, Gigabit passive optical network (GPON) is becoming a hot spot for study at present. How to carry the existing profitable SDH communication service over GPON networks in a cost-effective manner is an important problem that needs to be settled, in order that GPON is promoted and supported by applications.
In SDH services, Time division multiplexing (TDM) services at various rates play an important role. At present, in a GPON network, there are mainly the following two manners in which a TDM service is carried:
The first one is “TDM over GEM” manner, in which TDM service is mapped into a GEM (GPON Encapsulation method) frame. According to this manner, TDM service is encapsulated by using an unsteady length of GEM frame, and is transmitted limitedly between an OLT and an ONT; and because clock recovery is carried out in an adaptive manner, it is difficult for the clock recovery to meet requirements for controlling jitter and drift; and
The second one is “TDM over Ethernet” manner, in which TDM service is transmitted via an Ethernet frame. According to this manner, a code format based on Ethernet frame is required and a larger transmission bandwidth is needed, and therefore the cost of an ONU is increased.
For solving the above problems, an “SDH over GEM” manner is proposed, according to which SDH structure frame is used to implement carrying of TDM service in GPON network. At present, there are mainly the following two methods of implementing the “SDH over GEM”:
The first method is to use a modified SDH virtual container (VC) to carry TDM service. Basically, the method includes: modifying frame structures of SDH VC11 and VC12, mapping TDM services at T1 rate and E1 rate into the modified frame structures of VC11 and VC12 respectively, encapsulating each block of the modified VC11 and VC12 into a GEM frame, and transmitting TDM services via GEM frames. According to this method, it is required to modify the structure of a service frame for mapping, and an SDH frame can not be transmitted to an ONT directly; a structured transmission of TDM service cannot be accomplished because the structure thereof is not standard; and byte adjustment can not be achieved because a virtual container only provides a positive and reverse adjustment and filling bits.
The second method is to encapsulate various types of TDM services into frame structures of an SDH virtual tributary, in a payload form of ignoring data structures, or in a payload form of locking to data structure, or in a floating payload form of indicating data structure using a pointer; to encapsulate the virtual tributary into an Ethernet frame, and encapsulate the Ethernet frame into payload of a GEM frame, i.e., TDM service is carried in GPON network in “Ethernet over GEM” manner. Although in this method there is no need to modify the existing standards, TDM service has to undergo three mappings through VT, Ethernet and GPON. Thus this method is complicated in implementation and results in a high protocol overhead cost and larger bandwidth.